arXiv subjects

Masahito Mochizuki

Publications and source records attributed to Masahito Mochizuki.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic Skyrmion Interacting with Optical Skyrmion

Magnetic skyrmions (MSks) and optical skyrmions (OSks) embody topology in matter and in light, respectively. Here we investigate the interaction between a single MSk and an OSk beam. Three distinct nonlinear dynamical modes are identified: rotation, skipping, and trochoidal motion. By decomposing the optical driving force into gradient, orbital-angular-momentum, and spin-angular-momentum contributions, we clarify their respective roles of radial confinement, azimuthal drift, and precessional modulation. The skipping motion arises from the azimuthal asymmetry of the OSk beam and exhibits spatial selectivity originating from the magnetization-polarization coupling between the MSk and OSk. In three dimensions, the coupling acquires a propagation-dependent phase dominated by the differential Gouy phase, which yields $z$-asymmetric skipping trajectories. These results bridge topological particles and topological fields within a unified framework, offering helicity-selective and phase-programmable routes to optomagnonic control.

cond-mat.mes-hall↗

arXiv2026

Chiral Damping-Induced Chirality Switching and Control of Domain Walls in Antiferromagnets

We investigate the impact of chiral damping (CD) on current-driven domain-wall (DW) dynamics in antiferromagnets (AFMs). Asymmetric CD between sublattices generates off-diagonal components in the DW mass tensor, thereby coupling translational and rotational modes. When CD is modulated by an ac gate voltage via the Rashba spin-orbit interaction (RSOI), symmetric and asymmetric contributions induce oscillations in the DW velocity and tilt angle, respectively. A perturbative analysis yields explicit expressions for the oscillation amplitudes, in quantitative agreement with numerical simulations. Remarkably, even in the absence of Dzyaloshinskii-Moriya interaction (DMI), asymmetric CD enables chirality switching between Néel- and Bloch-type DWs. Finally, by exploiting the relativistic Lorentz contraction of the DW width at high driving currents, we propose an experimentally viable protocol to qualitatively and quantitatively extract the CD contribution. These results establish clear experimental signatures of CD in antiferromagnetic DW dynamics and demonstrate its potential as a control parameter for magnetic textures.

cond-mat.mes-hall↗

arXiv2026

Diffusion asymmetry of repulsive skyrmions in structured environment

The diffusion of matter in structured environments can give rise to emergent phenomena that do not occur in unstructured environments. Here, we report the asymmetric diffusion of magnetic skyrmions in structured chambers, where repulsive skyrmion-skyrmion and skyrmion-environment interactions play vital roles in their diffusive behavior. By fabricating an off-center asymmetric gate separating two chambers, skyrmions could demonstrate asymmetric diffusion through the gate, which depends on the gate symmetry, the gate opening width, and the skyrmion density. Although the diffusion is affected by the skyrmion density, the simulation outcomes are generally in line with rate equation solutions assuming time-independent diffusion rates. Diffusive skyrmions within the chamber can transiently form bonds through repulsive skyrmion-skyrmion interactions, leading to emergent rotational dynamics that is unique to interacting skyrmion systems. Our results uncover asymmetric diffusive behavior of skyrmions, offering insights that will appeal to the wide audience interested in magnetism, active matter, and statistical physics.

cond-mat.mes-hall↗

arXiv2026

Fate of Topological Dirac Magnons in van der Waals Ferromagnets at Finite Temperature

Dirac magnons, the bosonic counterparts of Dirac fermions in graphene, provide a unique platform to explore symmetry-protected band crossings and quantum geometry in magnetic insulators, while promising high-velocity, low-dissipation spin transport for next-generation magnonic technologies. However, their stability under realistic, finite-temperature conditions remains an open question. Here, we develop a comprehensive microscopic theory of thermal magnon-magnon interactions in van der Waals honeycomb ferromagnets, focusing on both gapless and gapped Dirac magnons. Using nonlinear spin-wave theory with magnon self-energy corrections and a T-matrix resummation that captures two-magnon bound states, we quantitatively reproduce temperature- and momentum-dependent energy shifts and linewidths observed experimentally in the gapless Dirac magnon material CrBr$_3$, even near the Curie temperature. Our approach resolves discrepancies between prior theoretical predictions and experiment and highlight the significant role of bound states in enhancing magnon damping at low temperatures. For gapped Dirac magnon materials such as CrI$_3$, CrSiTe$_3$, and CrGeTe$_3$, we find a thermally induced reduction of the topological magnon gap but no evidence of thermally driven topological transitions. Classical atomistic spin dynamics simulations corroborate the gap' s robustness up to the Curie temperature. Furthermore, we establish a practical criterion for observing topological gaps by determining the minimum ratio of Dzyaloshinskii-Moriya interaction to Heisenberg exchange required to overcome thermal broadening throughout the ordered phase, typically around 5%. Our results clarify the interplay of thermal many-body effects and topology in low-dimensional magnets and provide a reliable framework for interpreting spectroscopic experiments.

cond-mat.str-el↗

arXiv2026

Exact Solution for Current-Driven Domain-Wall Dynamics Beyond Lorentz Contraction in Antiferromagnets with Dzyaloshinskii-Moriya Interaction

We study current-driven domain-wall (DW) dynamics in antiferromagnets (AFMs) with Dzyaloshinskii-Moriya interaction (DMI). We obtain an exact analytical solution for spiral DW dynamics, applicable to both head-to-head DWs under bulk DMI and up-down DWs under interfacial DMI when the magnetic easy axis is aligned with the DMI vector. For the latter case experimentally relevant to synthetic AFMs with in-plane anisotropy, the solution predicts a constant DW velocity driven by nonadiabatic spin-transfer torque together with a steady rotation of the DW tilt angle induced by damping-like spin-orbit torque. Remarkably, the DW width shows unconventional current dependence, either pure elongation or contraction followed by elongation depending on damping and torque parameters, in sharp contrast to the Lorentz-type contraction known for antiferromagnetic (AF) DWs without DMI. These results provide an exact description of current-driven AF-DW dynamics and suggest experimentally accessible signatures of DMI-modified DW dynamics in synthetic AFMs.

cond-mat.mes-hall↗

arXiv2026

Circular dichroism in second- and third-harmonic generation in chiral topological semimetal CoSi

We theoretically investigate circular dichroism (CD) in second- and third-harmonic generation (SHG and THG) in the chiral topological semimetal CoSi. We demonstrate that both SHG and THG exhibit dichroic responses of order unity, while their robustness against spectral broadening is strikingly different. Specifically, while SHG-CD is strongly suppressed by dissipation, THG-CD remains robust over a wide frequency range. We show that this qualitative difference originates from the phase structure of the nonlinear current, where SHG-CD arises from subleading interference processes that are sensitive to dephasing, whereas THG-CD emerges already at the leading nonlinear order and is therefore protected against spectral broadening. As a result, THG-CD provides a robust probe of chirality encoded in nonequilibrium electronic dynamics. We further reveal non-monotonic frequency dependences and pronounced sensitivity of harmonic emission to the polarization state and crystallographic orientation of the driving field. Our results uncover a general mechanism for robust nonlinear chiroptical responses in noncentrosymmetric quantum materials and establish high-harmonic spectroscopy as a powerful probe of phase-resolved electronic dynamics.

cond-mat.str-el↗

arXiv2026

Janus skyrmion: Interfacial quasiparticle with two-faced helicity

Janus particles are functional particles with at least two surfaces showing asymmetric properties. We show at the interface between two magnetic regions with different antisymmetric exchange interactions, an alternative species of two-dimensional topological quasiparticles can emerge, in which different helicity structures can coexist. We name such an interfacial quasiparticle a "Janus skyrmion," in analogy to the Janus particle. As the Janus skyrmion shows helicity asymmetry, its size could vary with both the in-plane and out-of-plane magnetic fields. A vertical spin current could drive the Janus skyrmion into one-dimensional motion along the interface without showing the skyrmion Hall effect, at a speed which depends on both the in-plane spin-polarization direction and current density. Thermal fluctuations could also lead to one-dimensional random walk of a Brownian Janus skyrmion. This work uncovers unique dynamics intrinsic to interfacial quasiparticles with exotic helicity, which may be realized in interface-engineered magnetic layers.

cond-mat.mes-hall↗

arXiv2025

Nanofluidic logic based on chiral skyrmion flows

Particle-like chiral magnetic skyrmions can flow in nanotracks and behave like chiral fluids. Using interacting flows to perform logical operations is an important topic in microfluidics and nanofluidics. Here, we report a basic nanofluidic logic computing system based on chiral magnetic skyrmions flowing in parallel pipelines connected by an H-shaped junction. The flow behaviors could be manipulated by adjusting the spin polarization angle, which controls the intrinsic skyrmion Hall angle. We demonstrate that within certain range of the spin polarization angle, fully developed skyrmion flows could lead to fluidic logical operations, which significantly reduce the complexity of skyrmion logic as there is no need for deterministic creation, precise control, and detection of a single isolated skyrmion. Our results suggest that the chiral flow behaviors of magnetic quasiparticles may offer possibilities for spintronic and nanofluidic functions.

cond-mat.mes-hall↗

arXiv2025

Switching magnetic texture via in-plane magnetic field in noncentrosymmetric dipolar magnets: From skyrmions to antiskyrmions and nontopological magnetic bubbles

We theoretically investigate field-induced switching of magnetic topology in a nanodisk-shaped sample of noncentrosymmetric dipolar magnet in which the Dzyaloshinskii-Moriya interaction that stabilizes an antiskyrmion with $N_{\rm sk}$=+1 and the magnetic dipole interaction that stabilizes a skyrmion with $N_{\rm sk}$=$-1$ are in keen competition where $N_{\rm sk}$ is the skyrmion number. Our micromagnetic simulations demonstrate that the competition offers a unique opportunity to switch magnetic textures with distinct magnetic topology among the antiskyrmion ($N_{\rm sk}$=+1), elliptical skyrmion ($N_{\rm sk}$=$-1$), and nontopological bubble ($N_{\rm sk}$=0) in a deterministic manner by application of magnetic fields parallel to the sample plane. By calculating time and spatial profiles of energy contributions from respective interactions and magnetic anisotropy, we clarify the physical mechanism and properties of the observed field-induced topology switching phenomena. Our findings are expected to provide useful insights into the spintronic application of topological magnetism.

cond-mat.mes-hall↗

arXiv2025

Spontaneous magnon decays from nonrelativistic time-reversal symmetry breaking in altermagnets

Quasiparticles are central to condensed matter physics, but their stability can be undermined by quantum many-body interactions. Magnons, quasiparticles in quantum magnets, are particularly intriguing because their properties are governed by both real and spin space. While crystal symmetries may be low, spin interactions often remain approximately isotropic, limiting spontaneous magnon decay. Textbook wisdom holds that collinear Heisenberg magnets follow a dichotomy: ferromagnets host stable magnons, while antiferromagnetic magnons may decay depending on dispersion curvature. Up to now, relativistic spin-orbit coupling and noncollinear order that connect spin space to real space, were shown to introduce more complex magnon instability mechanisms. Here, we show that even in nonrelativistic isotropic collinear systems, this conventional dichotomy is disrupted in altermagnets. Altermagnets, a newly identified class of collinear magnets, exhibit compensated spin order with nonrelativistic time-reversal symmetry breaking and even-parity band splitting. Using kinematic analysis, nonlinear spin-wave theory, and quantum simulations, we reveal that even weak band splitting opens a decay phase space, driving quasiparticle breakdown. Additionally, d-wave altermagnets form a rare ``island of stability'' at the Brillouin zone center. Our findings establish a quasiparticle stability trichotomy in collinear Heisenberg magnets and position altermagnets as a promising platform for unconventional spin dynamics.

cond-mat.str-el↗

arXiv2025

Predicted versatile topological nodal magnons in Tb-based icosahedral quasicrystal 1/1 approximants

Using a recently-established band representation analysis, we discover two distinct types of topological nodal magnons in the real-space antiferroic ordering of whirling spin arrangements in the Tb-based icosahedral quasicrystal 1/1 approximants, both of which originate from a composite band (co-)representation $A\uparrow P_In\bar{3}(24)$ and its constituent elementary band representations. The first type is doubly-degenerate nodal line network and nodal planes associated with two-dimensional irreducible band representation, while the second type is a nodal line network due to accidental band inversions. Since our analysis, which relies solely on magnetocrystalline symmetry, is valid for a wide range of materials and spin textures belonging to the same magnetic space group irrespective of composition, these findings offer new universal insights into the research of Tb-based quasicrystal approximants as well as a contribution to broadening the range of topological magnon-hosting materials.

cond-mat.str-el↗

arXiv2025

Skyrmion Hall effect and shape deformation of current-driven bilayer skyrmions in synthetic antiferromagnets

The commonly believed absence of skyrmion Hall effect for topologically trivial magnetic skyrmions is reconsidered for bilayer skyrmions in synthetic antiferromagnets driven by spin-transfer and spin-orbit torques. Using a general Lagrangian formalism, we show that Bloch-type bilayer skyrmions acquire a finite Hall angle when driven by spin-orbit torque, while Néel-type skyrmions do not, in agreement with micromagnetic simulations. Both types of skyrmions exhibit current-induced elliptical deformation with minor and major axes aligned longitudinally and transversely to their velocity, respectively. A linear relation between velocity and longitudinal radius is derived with a coefficient proportional to the strength of spin-orbit torque. These effects are critical for antiferromagnetic skyrmion-based applications such as skyrmion racetrack memory. The Lagrange equations also reproduce the linear Hall angle-helicity relation reported by Msiska et al., Phys. Rev. Appl. 17, 064015 (2022). An intuitive explanation of the skyrmion Hall effect for arbitrary helicity based on the antiferromagnetic exchange torque is also provided.

cond-mat.mes-hall↗

arXiv2025

All-electric control of skyrmion-bimeron transition in van der Waals heterostructures

Two-dimensional van der Waals materials offer a versatile platform for manipulating atomic-scale topological spin textures. In this study, using first-principles and micromagnetic calculations, we demonstrate a reversible transition between magnetic skyrmions and bimerons in a MoTeI/In_2Se_3 multiferroic heterostructure. The physical origin lies in the reorientation of the easy axis of magnetic anisotropy, triggered by the reversal of ferroelectric polarization. We show that the transition operates effectively under both static and dynamic conditions, exhibiting remarkable stability and flexibility. Notably, this transition can be achieved entirely through electric control, without requiring any external magnetic field. Furthermore, we propose a binary encoding scheme based on the skyrmion-bimeron transition, presenting a promising path toward energy-efficient spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Theory of Photocurrent and High-Harmonic Generation with Chiral Fermions

We theoretically discover possible dc-current induction and high-harmonic generation from photodriven chiral fermions in B20-type semimetals irradiated with circularly polarized light as nonlinear optical responses with several unconventional properties. First, we find multiple sign changes of the induced bulk dc photocurrent as a function of light parameters, which is ascribed to the nature of asymmetric photon-dressed bands in chiral systems. Moreover, we observe a parity-dependent directivity of high-harmonic generation where the odd- and even-order harmonics have intensities only in directions perpendicular and parallel to the polarization plane, respectively, which can be understood from dynamical symmetry of the present photodriven chiral systems.

cond-mat.mtrl-sci↗

arXiv2025

Theoretically proposed controlled creation of Bloch-type skyrmions with spin-orbit torque in a chiral-ferromagnet/heavy-metal heterojunction

The creation and manipulation of magnetic skyrmions in magnetic bilayer heterostructures via spin-orbit torque have been intensively studied in spintronics because of their potential application as information carriers in next-generation magnetic memory devices. However, experimental attempts have not always been successful. In this paper, we theoretically elucidate the underlying reasons for these difficulties and propose a practical method to overcome them by employing magnetic bilayer heterostructures that incorporate a chiral ferromagnetic layer hosting Bloch-type skyrmions instead of the conventional ferromagnetic layer that hosts Néel-type skyrmions. Our micromagnetic simulations demonstrate that Bloch-type skyrmions can be controllably created in this system via spin-orbit torque exerted by a perpendicular spin current. This finding provides a promising platform and method for realizing skyrmion-based spintronic devices.

cond-mat.mes-hall↗

arXiv2025

Theoretical perspectives on optical control of magnetism in spin-charge coupled systems

In this article, we review recent theoretical research on photocontrol of magnetism in several spin-charge coupled systems. The control of magnetism with light has been a central issue in condensed-matter physics, attracting enormous research interest both for fundamental science and for technological applications. This field of research has developed rapidly in recent years along with the development of laser technology. However, because the direct coupling between the light magnetic field and magnetization via the Zeeman coupling is very weak in terms of the energy scale, it is, in principle, difficult to induce dramatic effects as far as this magnetic light-matter interaction is exploited. On the contrary, the interaction between the light electric field and electron charges has an energy scale two to three orders of magnitude larger than that of the magnetic interaction. Therefore, we may realize astonishing photoinduced physical phenomena and novel optical device functions by exploiting this electric light-matter interaction. Spin-charge coupled magnets, e.g., double-exchange magnets, multiferroics materials, and Rashba electron systems, in which spins and charges are strongly coupled through several kinds of mechanisms such as exchange interactions and spin-orbit coupling, are ideal systems for realizing this idea. Recent theoretical studies have revealed that it is possible to control, manipulate and switch the magnetization coupled to electron charges in these systems through exciting and/or driving them with light electric fields. The following three recent topics are discussed as examples of such theoretical studies, that is, photoinduced magnetic phase transitions in irradiated double-exchange models, highly efficient photoinduction of spin polarization in Rashba electron systems, and electromagnon excitations and their intense excitation effects in multiferroic materials.

cond-mat.str-el↗

arXiv2025

Proposed controlled creation and manipulation of skyrmions with spin-orbit torque

The physical mechanisms underlying current-driven skyrmion motion include the spin-transfer torque exerted by a spin-polarized horizontal electric current and the spin-orbit torque exerted by a perpendicular spin current. Each mechanism requires a specific sample geometry and structural configuration. Regarding current-induced skyrmion creation, skyrmions can be efficiently created at low current densities via spin-transfer torque when an electric current is applied to a nanotrack structure with a small notch. However, an effective and controlled method for skyrmion creation via spin-orbit torque in notched nanotracks has yet to be established. Here we theoretically propose a method for the creation, driving, and deletion of skyrmions in a three-terminal magnetic heterojunction with a notch. Our proposal offers valuable insights into the design of techniques for skyrmion creation and manipulation using spin-orbit torque, which is essential for technical applications of magnetic skyrmions as information carriers in next-generation spintronic memory devices.

cond-mat.mes-hall↗

arXiv2025

Photoinduced magnetic phase transitions in the cubic Kondo-lattice model

We theoretically study photoinduced magnetic phase transitions and their dynamical processes in the Kondo-lattice model on a cubic lattice. It is demonstrated that light irradiation gives rise to magnetic phase transitions from the ground-state ferromagnetic state to a three-dimensional antiferromagnetic state as a nonequilibrium steady state in the photodriven system. This phase transition occurs as a consequence of the formation of pseudo half-filling band occupation via the photoexcitation and relaxation of electrons, where all the electron states constituting the lower band separated from the upper band by an exchange gap are partially but nearly uniformly occupied. We also find that several types of antiferromagnetic correlations, e.g., A-type and C-type antiferromagnetic correlations, appear in a transient state of the dynamical phase transition. By calculating magnon spectra for the photodriven system, we argue that the instability to the A-type or C-type antiferromagnetic state occurs in the ferromagnetic ground state as a softening of the magnon band dispersion at corresponding momentum points depending on the light polarization. Our findings provide important insights into the understanding of photoinduced magnetic phase transitions in the three-dimensional Kondo-lattice magnets.

cond-mat.str-el↗